1. Field of the Invention
This invention relates to a device for detecting absence of a thread in a sewing machine. More particularly, the device is particularly suitable for use during high-speed operation of such a sewing machine.
2. Description of the Prior Art
The prior art has proposed, for example, a device for detecting absence of a bobbin thread whereby a breakage or other abnormal condition of the bobbin thread may be detected by a lever which intermittently contacts the bobbin thread which in turn continuously repeats a slacking and tightening motion in cooperation with the cyclic movement of the needle bar. Generally, such a lever cooperates with a limit switch or a proximity switch which may be energized by an increased travel of the lever when the associated bobbin thread is absent. The disadvantage associated with such device is that the bobbin thread is likely to be damaged by the resilient force of a spring which is applied thereto by way of the lever. In order to cope with this difficulty, if the spring force is attenuated, the moving lever will not be able to follow the machine rotation. To the contrary, if the spring force is strengthened, the lever again will not be able to follow the machine except when the machine is in high-speed operation.
It has also heretofore been proposed to employ a device for detecting absence of a bobbin thread without touching the thread, that is, using a photoelectric element which detects the rocking motion of the bobbin thread in a direction perpendicular to the center of the thread hole. The use of such photoelectric element has the disadvantage that there is a considerable loss in its thread detecting function because of the dust inevitably produced and drifting about the needle hole.